In Julia 1.10, interpolation is calling the expected show method:

julia> pkgid = Base.PkgId(Base.UUID("295af30f-e4ad-537b-8983-00126c2a3abe"), "Revise")
Revise [295af30f-e4ad-537b-8983-00126c2a3abe]

julia> "$pkgid"
"Revise [295af30f-e4ad-537b-8983-00126c2a3abe]"

Indeed. This PR breaks that. I'm not sure how to achieve both a usable repr and the nice interpolation version

The missing qualification of Base.UUID is apparently due to the UUID show method:

julia> Base.UUID("295af30f-e4ad-537b-8983-00126c2a3abe")
UUID("295af30f-e4ad-537b-8983-00126c2a3abe")

which should probably be defined as:

show(io::IO, u::UUID) = print(io, UUID, "(\"", u, "\")")

so that it uses the show machinery for the type name.

I'm not sure how to achieve both a usable repr and the nice interpolation version

Interpolation calls print, which by default calls the 2-argument show. What do you want the interpolation to look like? Shouldn't it look like repr?

Normally the 3-arg show verbose/pretty form should only be used for things like REPL display (and explicit requests for text/plain output). If someone wants the display version in a string they should call repr("text/plain", pkgid).

What I think would make sense and be helpful would be

julia> pkgid
Revise [295af30f-e4ad-537b-8983-00126c2a3abe]

julia> "$pkgid did something"
"Revise [295af30f-e4ad-537b-8983-00126c2a3abe] did something"

julia> "$(repr(pkgid))"
"Base.PkgId(UUID(\"295af30f-e4ad-537b-8983-00126c2a3abe\"), \"Revise\")"

julia> repr(pkgid))
"Base.PkgId(UUID(\"295af30f-e4ad-537b-8983-00126c2a3abe\"), \"Revise\")"

That's not the way string interpolation works with any other type of object AFAIK… I don't think it would be a good idea to make PkgId work differently.

If you want the 3-arg show version in string interpolation you do "$(repr("text/plain", pkgid)) did something"

Only ":windows: build i686-w64-mingw32" failed in 4s.
So, LGTM?

I was just reading through the loading.jl file in detail, and it seems like this failed to update most of the places that I think would be expected, such as $modkey or $req_key or isprecompiled. Do we fix those or revert this for v1.11, or is that intended?

Just fix them up I guess. This is still an improvement to make the show method follow the documentation of show and repr as in #52793 (comment).